The present invention relates to methods and apparatus for data communications and more particularly to novel methods and apparatus for the rejection of adjacent data carrier signals and potentially-interfering harmonics of a system-wide signal in a frequency-diversity-multiplex (FDM) data communications system.
Data communications systems necessarily require a high signal-to-noise ratio to assure implementation of minimal error (or "false data") rates. Frequently, such data systems operate in a hostile environment containing potentially-interfering signals which, if not suppresed by the methods utilized for transmission of data, will contribute significant magnitudes of interfering energy whereby the desired signal is lost in the "noise" generated thereby. One method known to alleviate the interference problem and, consequently, achieve a satisfactory signal-to-noise ratio, is to increase the amplitude of the transmitted signal, whereby the signal at a receiving end is sufficiently raised above the "noise" to realize relatively low error rates. In many applications, the use of a high transmitter energy is undesirable either because of the additional interference to other portions of the system or the surrounding environment from radiating transmitted energy, or because the physical characteristics of the transmission medium dictate against usage of increased power levels.
One such system, particularly adapted for use on commercial power lines (of the type carrying AC energy at a frequency in the region of 50-60 Hz.) for monitoring loads, providing load control, detecting and locating faults, providing transformer protection, automatic metering, two-way communications and the like, is described in U.S. Pat. No. 3,944,723, issued Mar. 16, 1976; U.S. Pat. No. 3,944,932, also issued Mar. 16, 1976; and allowed applications Ser. Nos. 529,998 and 529,999, filed Dec. 5, 1974, now respectively U.S. Pat. Nos. 3,973,740 and 3,973,087, both issued Aug. 3, 1976, all of which applications and patents are assigned to the assignee of the present invention and are incorporated herein by reference. At the outset, it should be understood and is most heavily emphasized that the "Power Line Access Data System" (PADS) disclosed in the four above-mentioned U.S. patents is but a single example of a data communication system to which the present invention may be applied. Any data communications system operating in a noisy environment, wherein at least some of the potentially-interfering signals are harmonically related to a system-pervasive master signal, may advantageously utilize the instant invention. In the PADS system a two-way data communication system is disclosed which utilizes power transmission lines as a transmission medium, with a multi-level "tree" system of repeaters each utilizing a plurality of carrier frequencies interspersed between harmonics of the power line frequency, for the transmission of data from one level of the "tree" to a next higher or lower level. Each carrier frequency (or "tone") is generated by an oscillator (32 or 37 of FIG. 4 of e.g., U.S. Pat. No. 3,944,723) phase locked by means of a frequency divider 34, 38 and a phase comparator 33, 36 to a submultiple of a high frequency clock signal provided by crystal oscillator. Each carrier frequency is chosen to lie approximately mid-way between a pair of sequential harmonics of the nominal line frequency, i.e. f.sub.c,n =(n + 1/2).multidot.f.sub.nom. where f.sub.nom. is the nominal (average) line frequency, e.g. 60.000 Hz. and f.sub.c,n is the frequency of the carrier between the n-th and (n+1)-st harmonics of f.sub.nom. Data is transmitted in digital format, i.e. a serial sequence of binary patterns, by on-off keying (OOK) of twelve possible tone frequencies, to present a combination of two selected tones of the twelve possible tones to signify the presence of a data bit in each part. The baud rate, i.e. bits per second, at which the digital signal is transmitted is determined by a local baud clock pulse rate derived at each individual station on the "tree" system from a station master clock 56.
Degradation of the signal-to-noise ratio occurs as the system-wide signal incrementally varies in frequency, e.g. a 60 Hz. power line frequency (having a standard stability of +0.06 Hz. maximum) and as the side bands of an adjacent keyed carrier (having the well-known (sin X)/X frequency spectrum) impinge within the relatively narrow bandpass of the system receiver.
It is desirable to utilize a synchronized main data detector means (such as the Integrate-and-Dump matched filter described at pages 275 et seq. of S. Stein and J. Jones, Modern Communications Principles (McGraw-Hill, 1967)) to average to zero the contributions of at least some of the potentially-interfering signals. In order to properly synchronize the main detector means, the data to be transmitted must be preceded by a synchronization sequence separately processed by unsynchronized means at the receiving end for acquiring this signal in the hostile environment. It is at the synchronization acquisition means that methods and apparatus reducing the effect of frequency shift in a system-wide signal and from the sidebands of adjacent data carriers are extremely desirable.